Two distinct-but-related issues affect the bad anatomy tag, as well as the related tags bad proportions and anatomical nonsense.
Ambiguity
The first issue is that bad anatomy is not always obvious. Optical illusion can hide even major goofs, yet it's only very rarely that taggers leave comments explaining what it is that's so anatomically nonsensical about an image.
Subjectivity
The second issue is that these tags sometimes seem to get subjectively applied to what is clearly intended as deliberate stylistic exaggeration:
I say "seem" because there may be things I'm not noticing in these—see the first issue—but the most obviously unrealistic features in all of these images are the girls' three sizes, so in the absence of any explanatory comment I can only assume that it's those proportions that are supposed to be bad.
I think taggers should at least be encouraged to leave comments explaining what bits of a character's anatomy are supposed to be bad. The tags themselves could also probably use a slight redefinition. anatomical nonsense currently seems to mean "bad anatomy but even more worser!" which is, to say the least, a pretty subjective distinction. I can see value in the tag for things like post #3302817, post #2661992, and post #3155822, but these seem sufficiently different in kind from regular bad anatomy that the implication doesn't hold.